1. FIELD OF THE INVENTION
The invention relates to regeneration of spent catalyst from an FCC unit.
2. DESCRIPTION OF RELATED ART NO.sub.x, or oxides of nitrogen, in flue gas streams from FCC regenerators is a pervasive problem. FCC units process heavy feeds containing nitrogen compounds, and much of this material is eventually converted into NO.sub.x emissions, either in the FCC regenerator (if operated in full CO burn mode) or in a downstream CO boiler (if operated in partial CO burn mode). Thus all FCC units processing nitrogen containing feeds can have a NO.sub.x emissions problem due to catalyst regeneration, but the type of regeneration employed (full or partial CO burn mode) will determine whether NO.sub.x emissions appear sooner (regenerator flue gas) or later (CO boiler).
Although there may be some nitrogen fixation, or conversion of nitrogen in regenerator air to NO.sub.x, most of the NO.sub.x emissions are believed to come from oxidation of nitrogen compounds in the feed.
Several ways have been developed to deal with the problem.
1. Feed hydrotreating, to keep NO.sub.x precursors from the FCC unit.
2. Segregated cracking of fresh feed.
3. Process approaches reducing NO.sub.x formation in complete CO burn mode via regenerator modifications. PA1 5. Stack gas cleanup isolated from the FCC process.
4. Catalytic approaches, using a catalyst or additive which is compatible with the FCC reactor, which suppress NO.sub.x formation or catalyze its reduction in a regenerator in complete CO burn mode.
The FCC process will be briefly reviewed, followed by a review of the state of the art in reducing NO.sub.x emissions. In addition, some of the factors forcing FCC operators to process worse feeds (with more nitrogen compounds) in hotter regenerators (which tends to increase NO.sub.x) in an ever more restrictive legislative environment will be discussed.